Here, "beginning" refers to "when we first believed." AT: "but I am writing to you what Christ commanded us to do when we first believed. (See: [[rc://en/ta/man/translate/figs-explicit]])
Conducting our lives according to God's commands is spoken of as if we were walking in them. The word "it" refers to love. "And he has commanded you, since you first believed, to love one another" (See: [[rc://en/ta/man/translate/figs-metaphor]])